Dual-Band Near-Field Probing Antenna for Enhancing the Performance of Dual-Band Shared-Aperture Linear-Polarized Phased Antenna Arrays

This paper presents a novel dual-band, single-feed near-field probing antenna (NFP) designed to provide feedback signals for in-situ calibration and linearization of millimeter-wave dual-band, shared-aperture, linear-polarized phased antenna arrays. The NFP is designed to deliver flat coupling magnitudes and constant group delays to surrounding antenna elements across both frequency bands, while adhering to the tight <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\lambda / 2$</tex> element spacing required at the high-frequency band. To validate its performance, a proof-of-concept prototype-a <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$2 \times 2$</tex> dual-band, shared-aperture, linear-polarized antenna array embedding the proposed NFP-was developed to operate within the <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$27-30 \text{GHz}$</tex> and <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$37-41 \text{GHz}$</tex> bands. S-parameter measurements confirmed the NFP's ability to effectively couple signals in both frequency bands, meeting the required group delay and frequency response constraints. Specifically, the measured coupling over 1 GHz segments (corresponding to the linearization bandwidth for a 200 MHz signal), the coupling magnitude variation remains below 1 dB across <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$26.5-28.5 \text{GHz}$</tex> and below 3 dB across 36-39 GHz. Furthermore, modulated signal measurements using two 200 MHz, 256-QAM OFDM signals demonstrated the NFP's capability in training a dual-band, NF-based digital predistortion (DPD) function. The DPD significantly enhanced the array's performance by compensating for non-linearities, achieving a 20 dB improvement in adjacent channel power ratio at 28 GHz and a 15 dB improvement at 38.5 GHz.